The present invention relates generally to a method for purifying carbon dioxide gas. In particular, the present invention relates to a method for removing sulfur dioxide (SO2) from carbon dioxide gas comprising SO2 as a contaminant. The method also removes NOx, if present as a further contaminant, from the carbon dioxide gas. The invention has particular application in the purification of crude carbon dioxide, e.g. flue gas from an oxyfuel combustion process in a pulverized coal fired power station in which sulfur containing carbonaceous or hydrocarbon fuel is combusted in a boiler to produce steam for electric power generation.
The term “SOx” means oxides of sulfur and includes SO2 and sulfur trioxide (SO3). The term “NOx” means oxides of nitrogen and includes primarily nitric oxide (NO) and nitrogen dioxide (NO2). NOx may comprise one or more other oxides of nitrogen including N2O, N2O4 and N2O3.
It has been asserted that one of the main causes of global warming is the rise in greenhouse gas contamination in the atmosphere due to anthropological effects. The main greenhouse gas which is being emitted, carbon dioxide (CO2), has risen in concentration in the atmosphere from 270 ppm before the industrial revolution to the current figure of about 378 ppm. Further rises in CO2 concentration are inevitable until CO2 emissions are curbed. The main sources of CO2 emission are fossil fuel fired electric power stations and from petroleum fuelled vehicles.
The use of fossil fuels is necessary in order to continue to produce the quantities of electric power that nations require to sustain their economies and lifestyles. There is, therefore, a need to devise efficient means by which CO2 may be captured from power stations burning fossil fuel so that it can be stored rather than being vented into the atmosphere. Storage may be deep undersea; in a geological formation such as a saline aquifer; or a depleted oil or natural gas formation. Alternatively, the CO2 could be used for enhanced oil recovery (EOR).
The oxyfuel combustion process seeks to mitigate the harmful effects of CO2 emissions by producing a net combustion product gas consisting of CO2 and water vapor by combusting a carbonaceous or hydrocarbon fuel in pure oxygen. This process would result in an absence of nitrogen (N2) in the flue gas, together with a very high combustion temperature which would not be practical in a furnace or boiler. In order to moderate the combustion temperature, part of the total flue gas stream is typically recycled, usually after cooling, back to the burner.
An oxyfuel process for CO2 capture from a pulverized coal-fired power boiler is described in a paper entitled “Oxy-combustion processes for CO2 capture from advanced supercritical PF and NGCC power plants” (Dillon et al; presented at GHGT-7, Vancouver, Sep. 2004), the disclosure of which is incorporated herein by reference.
Oxyfuel combustion produces raw flue gas containing primarily CO2, together with contaminants such as water vapor; “non-condensable” gases, i.e. gases from chemical processes which are not easily condensed by cooling, such as excess combustion oxygen (O2), and/or O2, N2 and argon (Ar) derived from any air leakage into the system; and acid gases such as SO3, SO2, hydrogen chloride (HCl), NO and NO2 produced as oxidation products from components in the fuel or by combination of N2 and O2 at high temperature. The precise concentrations of the gaseous impurities present in the flue gas depend on factors such as on the fuel composition; the level of N2 in the combustor; the combustion temperature; and the design of the burner and furnace.
In general, the final, purified, CO2 product should ideally be produced as a high pressure fluid stream for delivery into a pipeline for transportation to storage or to site of use, e.g. in EOR. The CO2 must be dry to avoid corrosion of, for example, a carbon steel pipeline. The CO2 impurity levels must not jeopardize the integrity of the geological storage site, particularly if the CO2 is to be used for EOR, and the transportation and storage must not infringe international and national treaties and regulations governing the transport and disposal of gas streams.
It is, therefore, necessary to purify the raw flue gas from the boiler or furnace to remove water vapor; SOx; NOx; soluble gaseous impurities such as HCl; and “non-condensable” gases such as O2, N2 and Ar, in order to produce a final CO2 product which will be suitable for storage or use.
In general, the prior art in the area of CO2 capture using the oxyfuel process has up to now concentrated on removal of SOx and NOx upstream of the CO2 compression train in a CO2 recovery and purification system, using current state of the art technology. SOx and NOx removal is based on flue gas desulphurization (FGD) schemes such as scrubbing with limestone slurry followed by air oxidation producing gypsum, and NOx reduction using a variety of techniques such as low NOx burners, over firing or using reducing agents such as ammonia or urea at elevated temperature with or without catalysts. Conventional SOx/NOx removal using desulphurization and NOx reduction technologies is disclosed in “Oxyfuel Combustion For Coal-Fired Power Generation With CO2 Capture—Opportunities And Challenges” (Jordal et al; GHGT-7, Vancouver, 2004). Such process could be applied to conventional coal boilers.
FGD scrubbing schemes typically involve reacting the acid gas, SO2, with an alkaline sorbent material at atmospheric pressure to produce sorbent-derived sulfite. Conventional alkaline sorbents include calcium carbonate (limestone), calcium hydroxide slaked or hydrated lime), and magnesium hydroxide. For example, the reaction taking place in a wet scrubbing process using limestone slurry producing calcium sulfite (CaSO3) can be expressed as:CaCO3(s)+SO2(g)→CaSO3(s)+CO2(g) 
Where the alkaline sorbent used is slaked lime slurry, the reaction taking place also produces calcium sulfite and can be expressed as:Ca(OH)2(s)+SO2(g)→CaSO3(s)+H2O(l) 
The reaction of magnesium hydroxide with SO2 producing magnesium sulfite may be expressed as:Mg(OH)s(2)+SO2(g)→MgSO3(s)+H2O(l) 
A solution of sodium hydroxide (NaOH), or caustic soda, may also be used as the alkaline sorbent.
Calcium sulfite is typically converted to the more commercially valuable calcium sulfate dihydrate (CaSO4.2H2O) or gypsum, by the following “forced oxidation” reaction which takes place in the presence of water:CaSO3(s)+½O2(g)→CaSO4(s) 
There are many examples of FGD schemes disclosed in the prior art that involve wet scrubbing with alkaline sorbents. An example of one such scheme is disclosed in U.S. Pat. No. 3,906,079 A. All of these schemes appear to operate at atmospheric pressure and produce only the sorbent-derived sulfite in significant quantities. The schemes involve additional processing steps to convert the sorbent-derived sulfite to the corresponding sulfate.
It has been suggested (Hack et al; “Advanced oxyfuel combustion leading to zero emission power generation”; 35th Int. Tech. Conf. on Clean Coal & Fuel Systems, Clearwater, Fla., USA; Jun. 6th-10th 2010) that natural acidic condensate drained from a carbon dioxide purification unit may be sent to a wet scrubber for capture and neutralization.
A process for the conversion of SOx/NOx, present in the stack gas of fossil fuel fired boilers, into concentrated sulphuric acid (H2SO4) and nitric acid (HNO3), respectively, has been developed by Tyco Labs., Inc. and is described in a report titled “Development of the catalytic chamber process for the manufacture of sulfuric and nitric acids from waste flue gases” (Keilin et al; Contract number PH86-68-75; Prepared for the US Environmental Protection Agency Office of Air Programs 1967 to 1969). The Tyco process is based on the lead chamber process for sulfuric acid manufacture. In this process, SO2 is oxidized to SO3 by reaction with NO2 (see Equation (a));SO2+NO2SO3+NO  (a)
This reaction is followed by dissolution of the SO3 in water to form sulfuric acid (see Equation (b));SO3+H2OH2SO4  (b)
The NO is reoxidized to NO2 by reaction with oxygen present in the flue gas (see Equation (c));2NO+O22NO2  (c)
In effect, NO2 acts as a gas phase catalyst for reaction (a) as NO2 is regenerated by reaction (c) from the NO produced in reaction (a).
This process would not normally be feasible at atmospheric pressure and with the low concentrations of NOx present.
A further problem would be the rather slow kinetics of the NO oxidation step. The Tyco process gets over this problem in two ways. First, it increases the NO2 concentration in the stack gas by a factor of about 100 by recycling an NO2-rich gas stream which is mixed with the stack gas prior to SO2 oxidation and H2SO4 production. The H2SO4 is recovered in a high temperature scrubber, which allows the bulk of the water vapor in the stack gas to pass through the unit without condensation, producing an acid of about 80% concentration. The NO2 and NO react with the sulfuric acid to form nitrosyl sulfuric acid so that about 90% of the NOx present in the flue gas is removed together with virtually all of the SOx (see Equation (d)).NO2+NO+2H2SO42NOHSO4+H2O  (d).
Secondly, the slow oxidation of NO to NO2 is speeded up by passing the nitrosyl sulfuric acid through a stripper tower which is swept by a small side-stream of the flue gas feed which provides the O2 needed for net NO oxidation to NO2. The oxidation reaction in the stripper tower is assisted by an active carbon catalyst which circulates in the liquid phase.
US 2007/0122328 A1 (granted as U.S. Pat. No. 7,416,716 B1) discloses the first known method of removing SO2 and NOx from crude carbon dioxide gas produced by oxyfuel combustion of a hydrocarbon or carbonaceous fuel, in which the removal steps take place in the CO2 compression train of a CO2 recovery and purification system. This process is known as a “sour compression” process since acid gases are compressed with carbon dioxide flue gas. The method comprises maintaining the crude carbon dioxide gas at elevated pressure(s) in the presence of O2 and water and, when SO2 is to be removed, NOx, for a sufficient time to convert SO2 to sulfuric acid and/or NOx to nitric acid; and separating said sulfuric acid and/or nitric acid from the crude carbon dioxide gas.
There is a continuing need to develop new methods for removing SOx and, where present, NOx from carbon dioxide gas, and particularly from crude carbon dioxide gas such as flue gas produced in an oxyfuel combustion process such as that involved in a pulverized coal-fired power boiler.